1. Field of the Invention
The present invention relates to a magnetic disc apparatus and in particular, to a magnetic disc apparatus for producing a pulse of a reproduction signal within a window.
2. Description of the Related Art
In a magnetic disc apparatus, a peak shift phenomenon can be seen. That is, a reproduction signal has a peak not stable and shifted. The range allowed for this peak shift is called a window margin, indicating the character of data read/write performance. In order to reduce the error rate indicating a bit mismatch between a recording signal and a reproduction signal, the window margin is adjusted in accordance with the performance of a magnetic disc medium. There have been suggested various methods to adjust this window margin according to a magnetic disc position or for respective heads.
For example, Japanese Patent Publication (unexamined) A3-95773 [1] discloses a method in which a multi-phase window is generated so as to select a window having an optimal phase difference with respect to a data signal.
Moreover, Japanese Patent Publication (unexamined) A9-180373 [2] filed by the same applicant as the present invention discloses a method of setting this window margin as follows. That is, a window position is shifted firstly in the plus direction and the minus direction, so as to determine error rates at the shifted positions. Next, a window margin is set according to the value of the error rate.
However, in the aforementioned conventional examples, the window position is modified at a constant interval and measurement is performed only once at each of the measurement points. Accordingly, when the same optimization is performed, there is a case that an optimal value is obtained at a position different from the previous optimization.
Especially, the inventor of the present invention has found in Document [2], that the window margin is not always maximum at the position where the parameter is optimal.
The inventor analyzed this problem and concluded that this problem is caused by the fact that each of the error rates for calculating the window margin (WDM) is measured only once at each window shift (WSF) point where the error rate easily fluctuates and that the optimization program including the window margin measurement is a procedure for varying one point in each parameter with an identical interval (step).